Frequency modulation



p l 20,1954 w. M. wees-ran, JR 2.676302 FREQUENCY MODULATION I FiledDec. 1'4, 1950 William jwfi'im ATTORNEY Patented Apr. 20, 1954 2,676,302FREQUENCY MODULATION William M. Webster,

J r., Princeton, N. J assignor to Radio Corporation of America, acorporationv of Delaware Application December 14, 1950, Serial No.200,717

. 1. This invention relates to frequency modulation, and moreparticularly to arrangements employing a single gaseous discharge deviceas an oscillator and as a variable capacitance for the same.

Recently, there have been devised gaseous discharge devices of the typewhich utilize an auxj iliary discharge and in which a. grid can be usedto controlor. modulate currents flowing from .a cathode to an anode.Devices of this type. are known. as .plasmatrons and their operation maybe summarized in the following way. In conventional hot cathode gaseousdevices the appliedanode potential serves a double function, the firstbeing the provision of the ionization needed for. space-chargeneutralization and the second being the provision of the. field neededfordrawing the required cathodev current. In the .plasmatronf? however,these functions are separated. As a result, the potentials needed fordrawing .given cathode currents are greatly reduced and, in addition,grid control of the anode current becomes possible.

-This separation of function is achieved by utilizing what is termed anauxiliary discharge to provide the space-charge neutralizing ionization.The ionization from this auxiliary discharge manifests itself bycreating a dense cloud of free ions and electrons in the region betweenthe main operating electrodes, viz., the hot cathode, the anode, and thegrid. This concentration of free ions and electrons is called a plasma.Because the ion and electron concentrations are approximately equal, theplasma is electrically neutral and acts as a good conductor for electroncurrents such as those from a, hot cathode. The higher the ion andelectron concentration the better will be the conducting properties ofthe plasma. This property of good conduction is responsible for forcingpractically all of an applied cathode-anode voltage to appearacross athin electron sheath at the anode. Then, inefiect, the cathode isimmediately adjacent-tothe anode.- This gives the plasmatron the highperveance associated with a vacuum tube having very closely-spacedelectrodes.

.If a negativeielectrodeis immersed in the plasma, a field-absorbinglayer called a "sheathforms around the electrode. layer-absorbs theelectric field so that the plasma remains isolated andcan retain itsfield-free nature. .Since the negative field allows only positive ionsto enterthis regiomthe sheath is called-a positive ion sheath. Y

"Ihe'thickness of a sheath such 'as described 7 Claims. (Cl. sea-1s)increases as the magnitude of the electrode potential increases. Thus, anegatively biased plasmatron grid will surround itself with a positiveion sheath that expands and contracts in accordance with an applied gridsignal. Then'the plasma cross-section, which connects the anode with thecathode through the grid openings, will have its area modulated. Thisaction, in turn, modulates the cathode-anode current by virtue of thefact that the conducting ability of the plasma, like that of otherconductors, depends upon its cross-sectional area. This plasmatron" gridcontrol can be pictured as a control of the area of the effectivecathode which appears adjacent to the anode. -In some cases, .the gridsheath will control the plasma density, and hence its conductingability, in regions close to the grid openings. This results in anadditional control action whichmight be viewed as a control of theemission capabilities of the effective cathode.

The above action is explained somewhat more fully in the copendingJohnson application, Serial No. 185,745, filed September 20, 1950.

Sincegaseous discharge devices of the abovedescribed .plasmatron typehave. smooth voltage-current characteristics similar to thosev of vacuumtubes and since a control electrode may be used to continuously controlthe flow of current through such devices, it becomes possible to utilizesuch devices in oscillatory circuits similar-to those in which vacuumtubes have hitherto normally been employed. The present inventionrelates to oscillators utilizing such plasmatronsgly 7 An object of thisinvention is to devise an arrangement for frequency modulatingplasmatron oscillators.

Another object is to provide an arrangement whereby plasmatronoscillators may be frequency modulated with a'high frequency deviationin an effective manner.

A further object is to devise an efiicient arrangement for frequencymodulating discharge device oscillators having a low plate impedance.

An additional object is to provide an arrangement for frequencymodulating an oscillator of the type which is particularly suited forlowvoltage, high-currentoutput.

The foregoing and other objects of thisin vention will be bestunderstood'from the following description of some exemplificationsthereof, reference being bad to the accompanyin; drawing. wherein: i

Fig. 1 is a circuit diagram or one arrangement also simultaneouslynected. to. an. intermediate:

whereby this invention may be put into effect; and

Fig. 2 is a similar diagram of modified arrangement.

The objects of this invention are accomplished, briefly, in thefollowing manner: A plasmatron gaseous discharge device has its threemain electrodes, consisting of the main cathode, grid and anode,connected in a more or less conventional oscillatory circuit of a typewhich is commonly used for three-electrode vacuum tubes. potential onthe auxiliary cathode ismodulated by a modulating input signal to varythe intensity of ionization of thezgas... variation in the thicknessof'the; positiveion sheath around the negative grid, thus varying thecapacitance afforded by thissheath. Since.

this latter capacitance is connected directly across the external tunedcircuit, variations of such capacitance produce correspondingvariationsin the.oscillationfrequency.

Fig. l is a circuit diagram whereby this invention may be carriedintoeffect. Referring to this figure, numeral. I. denotes a gaseous.dise charge device of the so-called plasmatron type, such as describedin .the aforementioned copendingapplication; it will be seen that. the.representation oithedischargedevice. l is quitesimh lar to that in:Fig.1 of, such application. Inside: the envelope of device; I amain cathode2 par tially surrounded; by a U-shaped control grid 3 and a.U-shapedanode. 4,..grid3 being located between cathode 2-.and anode 4.Opposite the. openends. of grid 3..and anodel is mounted a cylindricalfocusing. electrode. 5. An.v ionization.

or. auxiliary. cathodev 6: is. mounted .coaxially and. concentric-with.respect to :cylindrical; electrode 5.

Asindicated. in: Fig. 1,,electrode. 5. is. provided with an: opening Ithe. center of which; is an a, planev passing. throughthe axes of;auxiliary and main. cathodes. Band 2;-

For. further. details of; the constructioniof. gaseous. discharge devicel,.reference may be had to the. aforementioned; Johnson application.

Device. I .is proyidedwitha .gaseous.atmosphere prior. to. sealingtofi;.as .indicatedzimFig; 1..

In; the operation. of fplasmatroni" l. as .an; 05. cillatorv according.to; this. inventiom. focusinge electrode .5; is normally-:leit...free?or: floating? although it. is. to. be; understood; that; this. elemtrode-may be; connected to auxiliary.- cathode; 6 cathode 6.. is; connected.

if. desired. Auxiliary. through the secondary. 8 of; an inputaor;modulartionztransformer 9,-and. through=.a-..current limit-.- ingresistance 10 to the I l of L unidirectional.- potential, thepositiveside ofwhich'is grounded. 2, 3 and 4 are connected as a Hartleyoscillator. To .efiect: this-result, the :-lower end i of. the parallelL..C.. oscillatory circuita l2; consistingzof induc-- llgrisgroundedtance- I 3 .and. variable :condenser:

The

This. causes ;a..

negative side of a sources.

4 herently negative with respect to cathode 2 or ground. The output ofthe oscillator may be taken off by means of a winding l1 inductivelycoupled to inductance [3. The source H has been indicated as a simplebattery but it is, of course, to be understood that any suitable sourcemay be utilized.

The potential of source; H, whichlis .the D. C. potentialbetween-auxiliary cathode B and main cathode 2, is so selected oradjusted that it is sufficient to cause an ionizing discharge to appearbetween these two cathodes, cathode 2 being positive with respect tocathode 6. For example, thispotential. may be in the range of ap- Thepotential defierence between themain cathode 2 and anode 4 isforexamplelS volts and is too low to produce The .three .main electrodesand the anode 4 is also:grounded'.for oscillatory currents bycapacitance l5. Cathode. 2. is-conl3.and.operates at D. ode 4. isconnectedv to point.. on inductance: An-

unidirectional source, oi the magntitude of 15- volts, for. example.Grid 3*is connected t'd the upper end of inductance l3 through ai-parallel R. C.,cir cuit [6... which functions toestablish: on saidgrid,during operation, abias which-is negative with, respect tocathode 2onground. Al.--

ternatively,. this. biasing; arrangement-maybe omitted and grid 3 leftfree.toseekits-own.pee;

tential .during.operation, such potential being-duanode region as well.Thus,=.there. is a highly COD? ductive: path. from. the..- main: cathode.L 2 ithrouglr; the grid 3 to;-the ..main;anode:4;;

As ,the grid swings :aoout itSZbiZJSiEEDOtEIltifil the: main current iscontrolled. over a wide. range of. approximately. zerdto;200;oramorermilliamperes; in. the mannerspreviouslyx described imthesum. mary of operation: of? the: plasmatronf Since this grid control oftheinain .anodezcurrent; does. takeplace, the devicel maybeconnected-Lin. an. oscillatoryr circuit; similar to. vacuum tube=..oscil-.- latory circuits, .and.- such. device williunction: to:produce oscillatory energy in: acsimilar: 'manner: InIig. 1', thedevicexzl isconnected innaiilartley oscillator. circuit, such: devicethen functioning to produce. oscillatory energy in tank or tuned circuitl2, which; energy: may: bewabstracted. coil-J1 asoutputc According: tothis i ventiony.the frequency of operation of a-.plasmatronoscillatorsuch as that previously-described may be varied or modu lated'electronically-by varying-*thedntensity oi the ionization. orauxiliary-- discharge current.

When a negatively charged electrode such asgrid 3 is placed: in astrongly-ionized gas -at' low pressure itlbecomes surrounded by a sheathof" positiveions. This sheath:lia's-'a= certain oapaci-- tance, and by:analogy I with "a condenser of ordi' nary construction theplatesaraconstitutedby the electrode3 itself and the-outer surfacearthapositive ion-sheath: Sinoe 'elctrodesaZ, 3' and 4 are connectedto-t'uned circuit I 2-; it may-be seenthat thissheatli-capacitance-iscoupled to 'the' oscillator tuned I circuit.When-this capacitance is I increased, the oscillator frequencyisdecreased." while when: this capacitance is descreased, the oscillatorfrequency is increased; since the sheath-capacitance is essentially-'across tank 1 circuit i 2.

The positive ionsheath about grid il lias'aden-- nite'thickness,its'thickness-depending on, among-'- other things, the:intensity ofionization of the gas. Thus,:. about: the negatively charged: trode 3there is an electric: fieldi -Tl'liscfieldf:ex.-:-- tends out--ta-isuchr-aidistance thatritsisrexactly connected elece 1 aczacoe sinceelectrons cannot enter it and thepositive ion current is small. Thisregion extends between two conductive surfaces, one being the electrode3 carryingv a negative electron charge and the. other being the surfaceof the, sheath, whichis.

at the potential of the gas within the envelope, thatis, with gases atlow pressure, at approximately the potential of. the cathode 2. This gasis conductive since negative electrons are moving freely about in it. Assuch, this sheath is in reality a condenser. Sincethe distance betweenvthe plates of the condenser, that is, the thickness of the sheath,varies with the intensity of ionization of, the gas, as alreadyexplained, the capacitance of the condenser variesin accordance with theintensity of ionization of the gas. The capacitance of the condenser, ofcourse, varies in an inverse direction with the distance between thecondenser plates (the thickness of the sheath).

An input modulating signal is applied to the primary I 8 of transformer9. Such a signal, appearing in secondary 8, which is in circuit betweenauxiliary cathode 6 and main cathode 2, changes the current flowingbetween these two cathodes. This change of'current varies the intensityof ionization of' the gasin device I, it being remembered that maincathode 2 acts as the anode for auxiliary cathode 6. This variation inintensity of ionization causes a variation in the thickness of thepositive ion sheath around grid 3, as previously explained, and avariation in the capacitance afiorded or provided by this sheath. Thiscapacitance variation produces a corresponding variation in thefrequency of the oscillator. Thus, input signals applied to [8 result infrequency modulation of the plasmatron" oscillator disclosed.

To particularize, an input signal of such polarity as to increase thecurrent flowing between cathodes 6 and 2 increases the ionizationintensity in device I, decreasing the thickness of the ion sheath aroundgrid 3 and thereby increasing the capacitance provided thereby; thisdecreases the oscillator frequency. Conversely, an input signal of theopposite polarity decreases the ionization intensity, resulting in anincrease of oscillator frequency.

In a circuit according to the present invention which was actually builtand tested, it was found that the plasmatron oscillator was easilyfrequency modulated; in other words, the frequency deviation was high.This may be seen from the following two examples. At 17 megacycles,increasing the discharge or ionization current from one to twomilliamperes caused a frequency decrease of 500 kilocycles, with nochange in amplitude. At 70 kilocycles, increasing the discharge currentfrom one to six milliamperes caused a frequency decrease of 50kilocycles.

It is desirable to point out that the oscillator of this invention isparticularly adapted for modulation at audio frequency rates, up toabout kilocycles. The upper frequency limit of the the. field of thepositive. ions in "plasmatron determined.

vFig. 2 discloses a modified arrangement. In this figure, elements thesame as those of Fig. 1 are denoted by the same reference numerals. InFig. 2, instead of feeding the input modulating signal to auxiliarycathode 6 by way of a transformer. as in Fig. 1, the input signal is fedto such cathode by way of a vacuum tube 1 9 which acts in effect as aseries variable impedance between source II. and auxiliary cathode 6.The negative side of source ,H is connected to the cathode 20 of tube I9and the positive side of this source is grounded. The input modulatingsignal is applied between the control grid 2| of tube l9 and ground,while the anode of tube !9 is connected to auxiliary cathode 6. Thus,,tube 19 pro: vides a series variable impedance between source H andcathode ii, and by theapplicationof a signal to control grid 2! thisimpedance is variedto thereby vary the yoltage between cathodes 6 and 2to vary thev intensity of ionization of the gas. This results in avariation of the oscillator frequency, as previously described inconnection with Fig. 1.

What I claim to be my invention is: r 1. A frequency modulationarrangement, comprising a gaseous discharge device, means for ionizingthe gas in said device, said device having a plurality of electrodesincluding anode and cathode electrodes and also acontrol electrodeoperating at a potential negative with respect to .the potential of saidgas, therebyforming a positive ion sheath about said control electrode,meansfor supplying said electrodes with operating'potentials, a tunedcircuit; means coupling said anode, cathode and control electrodes tospaced points on said circuit, thereby to develop oscillations in saiddevice and said circuit, and means for controlling said ionizing meansby a modulating signal to modulate the intensity of ionization of saidgas and thereby also the thiCk-.

ness of said sheath and the provided thereby.

2. A frequency'modulation arrangement, comprising a gaseous dischargedevice having a plurality of electrodes including anode, cathode andcontrol electrodes, means for ionizing the gas in said device,connections for supplying said electrodes with operating potentials, atuned circuit, means coupling said anode, cathode and control electrodesto spaced points on said circuit, thereby to develop oscillations insaid device and said circuit, and means effective on said first-namedmeans for varying the intensity of ionization of the gas in said device,to thereby vary the effective capacitance provided by said controlelectrode.

3. A frequency modulation arrangement, comprising a gaseous dischargedevice having a plurality of electrodes including anode and cathodeelectrodes and also a control electrode operating at a potentialnegative with respect to the potential of the gas in said device,thereby forming a positive ion sheath about said control electrode,connections for supplying said electrodes with operating potentials, atuned circuit, means coupling said anode, cathode and control electrodesto spaced points on said circuit, thereby to develop oscillations insaid device and said circuit, and means for varying the intensity ofionization of said gas when ionized, to thereby vary the thickness ofsaid sheath and the effective capacitance provided thereby.

4. A frequency modulation arrangement, coineffective capacitance as anoscillator has not as yet been agewaca:

p i ing a: gaseous disch'argeadevicemaving: a plurrality of electrodesincluding anode, cathode and.

control' electrodesameans for ionizingvtheigas'in saidedevice;connections for supplying said; elec trodes with operating potentials,atunedzcircuit,

means coupling. said: anodcycathode and. control. electrodes .to spaced"pointspn'said-circuit; there. by to developoscillations in. saidi deviceandsa-id'circuit; and -means-i'or controllingsaidvionizeatmodulatingsignal: to: modulate. the :intensity of ionizatiomofsaid-gas and there--by alsothe eire'oti vecapacitance provided by ing meansiby said controlelectrode.-

5. A-frequency modulation arrangement comrdischarge device; means 1 f orprising: a gaseous establishing an ionizing discharge in the gas in-saiddevice, said: means including "an electrom emitting cathode,-

trode-and a control electrode, in said device; means for supplying saidelectrodes with operating potentials, a tuned circuit;

to spaced points on said circuit,-.thereby to develop oscillationsinsaid device-and" said circuit; and means forvary the intensity of Jionization :of said gas.

62 A frequency modulation arrangement-com prising: agaseous-dischargedevice; means for: ionizing discharge in the gasestablishing anin said device, said meansincluding anelectron emittingcathode, another-r cathode, and means for establishing anionizingxcurrent between said cathodes; a -plurality of electrodesimsaiddevice, said plurality including: saidiv anodeelectrode, and: also"including respect to the potential of said gas, thereby forming a.positive trode;-- meansfor. supplying said i electrodes 1 with operatingpotentials, a tuned circuit means'coupling said anode, other cathode andcontrol .elec-;

trodes to" spaced points-om said circuit, thereby anothercathode, and"means for establishing .an -ionizing current between said' cathodes aplurality-oi electrodes; including said other-"cathode and alsoincluding an anode e1ec+ meanscoupling said anode, other cathode and'controlelectrodes varying said ionizing currentv to other" cathode, an.a control. electrode operating at a potential: negative with:

ion sheathabout said. control. elecin said device,

a: coupleasaith'iomsheathrtossaid' circuit'land tm device" and'l saidcir cult; and: means forwvaryingi. said ionizing cur rent to vary.theiintensitys ofnionization-ofsaid gas and thereby; alsozthetthiclmessof-saidrsheath and the 1 effective capacit'anceeprovided; thereby;-

1; A :.frequency.= modulation arrangement coma prising: a gaseous?discharge'z'device; means for. establishing: ion'iz'mg discharge the:gas said means including anelect'rom emitting cathode; another? cathode;and: means for: establishingzanzionizing current thetweenisaid' saidcathodesg: a. plurality with respect to thecpotential .of'sald gas;thereby forming a positive. ion sheath about said control electrode;means forfisupplyinglsaidi electrodes with operating potentials, 1a.tunedv circuit; means" coupling said anode; other cathode. and.-controlv electrodes to spaced points on saidcircuit ,5 therebytocouplesaid ion sheath to said circuit: andrto develop oscillations in:said 1 device. and' said: sir-- cult; and means for controlling said.ionizing: current by a modulating signal tomodulateuthe intensity ofionizationzof: said gas and. thereby also the thickness ofisaidisheathcand the: ei.-- iective capacitance providediithereby;

References. Citedin; the file-of. this; patent.

UNITED STATES- PA-TENTS of: electrodes in said device;said:plurality?includingisaid other." oath--- I ode, an anode-electrode;andialso' including acon:-

trol electrode: operating: at a: potential negative:

